The present invention relates generally to a position detector, and, more particularly, to a position detector that detects a position of an object, such as a reticle, with high precision in an exposure apparatus used to expose various devices, such as a semiconductor chip, e.g., an IC and an LSI, a liquid crystal display, and a thin film magnetic head. The present invention is suitable for an exposure apparatus that utilizes extreme ultraviolet (“EUV”) light as an exposure light source.
A reduction projection exposure apparatus has been conventionally employed, which uses a projection optical system to project a circuit pattern on a mask (or a reticle) onto a wafer, etc., to transfer the circuit pattern, in manufacturing a fine semiconductor device, such as a semiconductor memory and a logic circuit in the photolithography technology.
The transferable minimum critical dimension (or the resolution) of the projection exposure apparatus is proportionate to a wavelength of the exposure light, and inversely proportional to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Along with recent demands for finer semiconductor devices, a shorter wavelength of ultraviolet light has been promoted in the exposure light source from an ultra-high pressure mercury lamp (i-line with a wavelength of approximately 365 nm) to a KrF excimer laser (with a wavelength of approximately 248 nm) and an ArF excimer laser (with a wavelength of approximately 193 nm).
However, lithography using ultraviolet (“UV”) light has a limit to satisfy the rapidly promoting fine processing of a semiconductor device, and a projection optical system that uses the EUV light with a wavelength of 10 to 15 nm shorter than that of the ultraviolet, which is referred to as an “EUV exposure apparatus” hereinafter, has been developed to efficiently transfer a very fine circuit pattern of 0.1 μm or less.
As the light absorption in a material remarkably increases in a wave range of the EUV light, it is impractical to use a refractive element, which is applicable to the visible and UV lights. In other words, there are no glass materials usable for the wave range of the EUV light. Accordingly, a reflective element or a mirror, e.g., a multilayer coating mirror, is used for the EUV exposure apparatus. In addition, a reflection reticle is used, which makes a pattern to be transferred, of an absorber on a mirror.
On the other hand, the improved resolution also demands highly precise alignments between the stage and the reticle, between the wafer and the stage, and between a reticle and a wafer in the projection exposure apparatus. The projection exposure apparatus includes a position detector for detecting an alignment mark formed on the reticle, etc., which is so-called an alignment optical system or an alignment scope.
For example, a reticle alignment between a reticle reference mark that has been precisely positioned relative to the apparatus body and an alignment mark on the reticle includes the steps of illuminating the reticle reference mark and the alignment mark using the alignment light that has a different wavelength from that of the exposure light, forming images of the lights that transmit the marks on an image pickup device, detecting a positional offset between the reticle and the apparatus body based on the positional relationship of the images of both marks, and conducting an alignment between the reticle and the apparatus body. See, for example, Japanese Patent Application, Publication No. 2002-353099.
However, the reticle alignment in the EUV exposure apparatus uses the reflection reticle and, thus, cannot detect the images of both marks, because the alignment light does not transmit through the reticle reference mark and the alignment mark.
In addition, the reflection reticle and the multilayer coating mirror are optimized so that they exhibit high reflectance to the EUV light's wavelength and do not exhibit sufficient reflectance to the alignment light as the non-exposure light. For example, in the through-the-lens (“TTL”) alignment that detects the alignment mark on the reflection reticle via the projection optical system using the non-exposure light, the alignment mark's reflectance is so low for the alignment light that the detected mark signal may have a lowered contrast.
Although it is conceivable to detect the alignment mark on the reflection reticle without intervening the projection optical system, using a particular wavelength of the non-exposure light might cause the mark signal to be undetectable due to a lack of contrast, since the pattern on the reflection reticle has guaranteed reflectance and an absorption factor to the exposure light, as discussed above. A mark signal with a low contrast does not meet a requirement of the highly precise position detection or ultimately makes the alignment mark itself undetectable.
There is a demand for a position detector and a position detecting method suitable for highly precise detections of a position for a reflective optical element.